Bokuji Komiyama

Wireless secret key generation exploiting reactance-domain scalar response of multipath fading channels

We describe a secure communication scheme that uses the random fluctuation of the natural environment of communication channels. Only the transmitter and the receiver share the communication channel characteristics. From reciprocity between a transmitter and a receiver, it is possible for them to share one-time information of their fluctuating channel. This can provide a secret key agreement scheme without key management and key distribution processes. In this paper, we propose a new secret key generation and agreement scheme that uses the fluctuation of channel characteristics with an electronically steerable parasitic array radiator (ESPAR) antenna. This antenna, which has been proposed and prototyped, is a smart antenna designed for consumers. Using the beam-forming technique of the ESPAR antenna, we can increase the fluctuation of the channel characteristics. From experimental results, we conclude that the proposed scheme has the ability to generate secret keys from the received signal strength indicator (RSSI) profile with sufficient independence.

Superconducting properties and crystal structures of single‐crystal niobium nitride thin films deposited at ambient substrate temperature

Single‐crystal niobium nitride (NbN) thin films were fabricated at ambient substrate temperature so that photoresist lift‐off techniques could be used in fabricating Josephson tunnel junctions. In this article, we describe the superconducting properties and crystal structure of the NbN films. Even though the substrates were not heated, the NbN films had excellent superconducting properties: a high Tc of 16 K, low normal‐state resistivity (ρ20=62 μΩ cm), and residual resistivity ratios RRR=ρ300/ρ20 above one. The film structures, which were investigated by x‐ray diffraction, electron diffraction patterns and transmission electron micrographs, show a single‐phase orientation without columnar and granular structures. We have found that the superconducting properties depend on the lattice parameter, and the best films had a lattice parameter of 0.446 nm. NbN/AlN/Nb tunnel junctions were fabricated to measure the superconducting energy gap of the NbN films. We estimated the energy gap ΔNbN to be 2.6 meV and th...

HIGH CRITICAL CURRENT DENSITY NBN/ALN/NBN TUNNEL JUNCTIONS FABRICATED ON AMBIENT TEMPERATURE MGO SUBSTRATES

NbN/AlN/NbN tunnel junctions are fabricated at ambient temperature on MgO substrates, and a critical current density of 8 kA/cm2 is obtained in junctions with 1.5–nm–thick AlN barriers. Even though the NbN/AlN/NbN trilayers are deposited without intentional heating, the junctions show a large gap voltage (Vg=5 mV), sharp quasiparticle current rise (ΔVg=0.16 mV), and small subgap leakage current (Vm=25 mV and Rsg/RN=9). This report shows that high‐quality NbN/AlN/NbN tunnel junctions can be made at ambient substrate temperature.

Penetration depth measurements of single‐crystal NbN films at millimeter‐wave region

The zero temperature penetration depth λ(0) of single‐crystal NbN thin films has been measured at 82 GHz using an open resonator. The NbN films used in this study have a (200) single‐crystal structure and Tc and ρ20 (normal state resistivity at 20 K) are 16 K and 62 mΩ cm, respectively. λ(0) was obtained by applying a least‐squares fit to the change of penetration depth using the BCS theory. We obtain λ(0)=194 nm, which shows good agreement with the calculated value from Tc,ρ20 and Δ(0).

Submillimeter wave responses in NbN/AlN/NbN tunnel junctions

The submillimeter wave frequency responses of NbN/AlN/NbN tunnel junctions have been investigated using quasioptical coupling techniques and an optically pumped far‐infrared laser. A two junction array was integrated at the feeds of a bowtie antenna. Measurements on the junctions were made by monitoring photon‐assisted tunneling steps induced in the current–voltage characteristics. These n=±1 photon steps were clearly observed with irradiation at 303, 594, 693, and 762 GHz. The results suggest it will be possible to realize submillimeter wave mixers by using these junctions.

Experimental Proof of Electrically Invisible State of Inductively Loaded Dipole and Proposal of Electrically Invisible Meander-Lines

We examine that an inductively loaded dipole becomes electrical invisible showing that the influence on a nearby antennas input impedance and on its radiation pattern (scattering) are minimized. We also verify that the direction of the current reverses on the electrically invisible dipole so that the integral of the dipole current becomes zero. By making an inductive load with a distributed constant line, we fabricate an electrically invisible dipole without using chip devices and evaluate its performance. Furthermore, we propose electrically invisible conductor lines formed by connecting these electrically invisible dipoles in series using open stubs as connectors, and analytically examine their invisible state

Series-Connected Josephson-Effect Mixers For Millimeter-Wave Radiation

Millimeterwave detectors have been made of a series-connected array of Josephson junctions. Each junction of the array responds in phase to radiation and therefore the series-connected array as a whole behaves as if it were a single Josephson junction. When the Josephson junctions have a high critical current density, the IF outputs have sharp peaks at the voltages V = 2NΔ/ne where N is the number of the Josephson junctions in series connection and n is an integer. The subharmonic energy gap structures in voltage-current curves of Josephson junctions come from the Andreev-reflection-assisted-tunneling according to the recent Arnold theories.

Properties of submillimeter wave responses in NbN/AlN/NbN tunnel junctions

We have investigated the submillimeter wave frequency responses of NbN/AlN/NbN tunnel junctions using quasi-optical coupling techniques and an optically-pumped far-infrared laser. A two junction array was integrated at the feeds of a bow-tie antenna. Measurements on the junctions were made by monitoring photon-assisted tunneling steps induced in the current-voltage characteristics. These n= ± 1 photon steps were clearly observed with irradiation at 303, 594, 693 and 762 GHz. The results suggest it will be possible to realize submillimeter wave mixers by using these junctions.

Josephson Array Oscillators in the Millimeter Wave Region

We have fabricated and tested Josephson array oscillators. The oscillators were formed by 20 Cu shunted Nb/AlOx/Nb tunnel junctions and a Nb microstrip line. The microstrip line was separated from the Nb ground-plane by a 1 µm-thickness layer of SiO dielectric, and the junctions were placed at intervals of one wavelength along the microstrip line. At resonance, all junctions are seen as a quasi-lumped array. The power of the oscillator was detected either by a Nb/AlOx/Nb tunnel junction or a shunted tunnel junction connected to the oscillator by a microstrip-line on the same substrate. Shapiro steps and photon assisted tunneling steps in the detector were clearly observed. The power delivered to the detector was about 1.9 µW at 312 GHz.

Superconducting Properties of Single-crystal NbN Thin Films Deposited at Ambient Substrate Temperature

Single-crystal NbN thin films were fabricated at ambient substrate temperature. The NbN films have excellent superconducting properties: high T c of 16 K, low normal-state resistivity (ρ20 = 62 μΩ cm), and residual resistivity ratios RRR = ρ300/ρ20 above one. The crystal structure was investigated by x-ray diffraction characteristics, electron diffraction pattern and transmission electron micrograph. NbN/AlN/Nb tunnel junctions have been fabricated to measure the superconducting energy gap of the NbN films We estimated the energy gap ΔNbN to be 2.6 meV and the magnetic penetration depth λNbN to be 202 nm.